EK18 is a mutant of trp repressor and is cal a "super-repressor" since it represses the transcription of genes responsible for typtophan synthesis at a lower level of tryptophan molecules in the cell. Its molecular weight is about 12.5 kDaltons per monomer and it exists primarily as a dimer. However, when the salt concentration is lowered, or the protein concentration is increased, oligomerization occurs between dimers. The protein-protein interaction seems to play a role in the mechanism of repression. In addition to the corepressor concentration, this oligomerization may act to fine tune the gene regulation. Several studies have been done using fluorescence spectroscopy to investigate the oligomerization, but no structural studies have been done to date. NMR studies of this oligomerization will tell how the two dimers interact (i.e., whether the tetramerization is specific or nonspecific, or whether is it symmetric or unsymmetric).